Plastic film or foil is commonly produced by blowing air into a molten plastic tube extruded from an annular opening of a blown film extruder die and cooling the extruded plastic as it expands into a bubble. Cooling of the molten plastic bubble is done by blowing air from an air ring external to the melt or through a combination of simultaneous external and internal cooling. Internal cooling can be done only with a die of a large diameter, where there is sufficient space available inside the annular opening of the die for passage of air into and out of the bubble. The cooled expanded tube is then passed in between two collapsing boards which form an acute angle before being taken up by two nip rolls. The nip rolls press the tube into a two layer web which is subsequently passed to a motorized winder.
As the flattened tube is taken up by nip rolls, a small amount of air is trapped between the two layers of film. Air from the blown film is thus slowly removed and this causes the diameter of the tube to diminish slowly, thus reducing the layflat width from the desired standard. The loss of air is greater for gusseted film tube and zipper locking types of tube. In locations where the day and night temperatures vary by a significant margin, the tube size will also vary unacceptably.
For large dies with internal cooling, the balancing of air flowing into and out of the bubble is absolutely necessary to maintain the bubble size. This flow balancing device also functions as a film tube diameter control system. Such a device is described in U.S. Pat. No. 4,140,460 to R. A. Carlsen. The air flow balancing control provides a proximation control method for tube diameter. To such a tube diameter control design, a layflat width measurement and associated computerized detection apparatus was added by H. Halter in U.S. Pat. No. 4,189,288.
These devices were designed for large capacity machines or machines for producing large diameters. This does not solve the problem of controlling tube diameter in small machines which use the small dies.
For small die machines, existing quality control methods make use of the measurement of the layflat width downstream of the nip rolls after the tube has been collapsed and flattened into a two layer web by the nip rolls. There is a time delay such that while the film tube is being expanded, a measurement downstream still reflects the former width. Similarly, when the film tube is shrinking, downstream measurement still incorrectly reflects the "right" width. Such a method will produce a constantly fluctuating width of film unless a complex correlation is made with the film speed, and unless a position follower is installed to follow the movement of film from side to side. The cost of this system is often unacceptably high.
The most logical method is to measure or detect the film tube diameter during the forming process. This method is direct and instantaneous. However, the bubble or the film tube itself is very soft. Air pressure inside the tube is low, about one inch of water, i.e. 0.036 psi (pounds per square inch) or less above ambient, depending on the thickness and material used. In addition, due to the blowing air, the tube is constantly moving or is in slight vibration. These factors have prevented simplified direct measurement solutions in small die systems prior to the mechanism of the present invention.